A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In device manufacturing methods using lithographic apparatus, an important factor in the yield, i.e., the percentage of correctly manufactured devices, is the accuracy within which layers are printed in relation to layers that have previously been formed. This is known as overlay and the overlay error budget will often be 10 nm or less. To achieve such accuracy, the substrate should be aligned to the mask pattern to be transferred with great accuracy.
Currently, alignment marks and/or overlay targets are defined in back end of line metal layers without considering a phase depth of the respective mark. Layer thicknesses used by chip manufacturers are fixed by design and extremely hard to change in favor of an improved alignment mark performance. In some cases, typical phase depth values are such that low signal strengths are measured, which renders performing a sufficiently accurate alignment measurement more difficult. In such cases, switching the wavelength with which the alignment is performed may improve the performance. However, such wavelength switching is not always possible, and may not be sufficient. Additionally, marks may operate in a regime where aligned position is very sensitive to phase depth fluctuations. This may also hamper accurate alignment.